This application claims priority to an application entitled xe2x80x9cDemodulation Apparatus and Method in a Communication System Employing Multi-Level Modulationxe2x80x9d filed in the Korean Industrial Property Office on Aug. 13, 2001 and assigned Serial No. 2001-48758, the contents of which are herein incorporated by reference.
1. Field of the Invention
The present invention relates generally to a demodulation apparatus and method in a communication system employing multi-level modulation, and in particular, to an apparatus and method for calculating input soft values of a channel decoder in a demodulator for a communication system employing 16-ary QAM (Quadrature Amplitude Modulation).
2. Description of the Related Art
In a data communication system, when a signal encoded by a channel encoder is modulated using 16-ary QAM, a typical multi-level modulation used to increase spectral efficiency, a demodulator in a receiver requires a mapping algorithm for generating soft values (or soft decision values) corresponding to output bits of the channel encoder from a 2-dimensional signal comprised of an in-phase signal component and a quadrature-phase signal component, in order for a channel decoder in the receiver to decode the modulated signal through soft decision decoding.
The mapping algorithm is classified into a simple metric procedure proposed by Nokia and a dual minimum metric procedure proposed by Motorola, and both algorithms calculate LLR (Log Likelihood Ratio) for the output bits and use the calculated LLR as an input soft value of the channel decoder.
The simple metric procedure, a mapping algorithm given by modifying a complex LLR calculation formula into a simple approximate formula, has a simple LLR calculation formula, but LLR distortion caused by the use of the approximate formula leads to performance degradation. The dual minimum metric procedure, a mapping algorithm of calculating LLR with a more accurate approximate formula and using the calculated LLR as an input soft value of the channel decoder, can make up for performance degradation of the simple metric procedure to some extent. However, compared with the simple metric procedure, this procedure needs increased calculations, thus causing a considerable increase in hardware complexity.
It is, therefore, an object of the present invention to provide an apparatus and method for simplifying obtaining of input soft values of a channel decoder, calculated by the dual minimum metric procedure, without a mapping table or complex processing needed to obtain a minimum distance value with a received signal in a demodulator for a communication system employing 16-ary QAM.
To achieve the above and other objects, there is provided a method for demodulating a received signal in a data communication system employing a modulation technique for dividing an output sequence of a channel encoder into 4 bits and mapping the bits to a specific one of 16 signal points having an in-phase component Xk and a quadrature-phase component Yk. The method comprises deciding a soft value Zk of a third demodulated symbol by subtracting a distance 2a between two demodulated symbols on the same axis of a mapping table from a level |Yk| of the quadrature-phase component Yk; setting a first variable xcex1 to xe2x80x9c0xe2x80x9d if the soft value Zk has a negative value, setting the first variable xcex1 to xe2x80x9cxe2x88x921xe2x80x9d if the Zk has a positive value and the quadrature-phase component Yk has a negative value, and setting the first variable xcex1 to xe2x80x9c1xe2x80x9d if the Zk has a positive value and the quadrature-phase component Yk has a positive value; determining a soft value of a fourth demodulated symbol by calculating Yk+xcex1*Zk using the quadrature-phase component Yk, the soft value Zk and the first variable xcex1; calculating a soft value Zxe2x80x2k of a first demodulated symbol by subtracting the distance 2a between two demodulated symbols on the same axis of a mapping table from a level |Xk| of the in-phase component Xk; setting a second variable xcex2 to xe2x80x9c0xe2x80x9d if the soft value Zxe2x80x2k has a negative value, setting the second variable xcex2 to xe2x80x9cxe2x88x921xe2x80x9d if the Zxe2x80x2k has a positive value and the in-phase component Xk has a negative value, and setting the second variable xcex2 to xe2x80x9c1xe2x80x9d if the Zxe2x80x2k has a positive value and the in-phase component Xk has a positive value; and determining a soft value of a second demodulated symbol by calculating Xk+xcex2*Zxe2x80x2k using the in-phase component Xk, the soft value Zxe2x80x2k and the second variable xcex2.